1. Field of the Invention
The invention relates generally to lightweight microwave structures and, more particularly, to lightweight microwave structures having excellent thermomechanical stability, and a method of making the same.
2. Brief Description of the Related Art
Due to a desire to minimize payload weight, and thereby reduce costs, it is preferable to use lightweight components whenever possible in the construction of high-power communications satellites. Furthermore, due to the extreme operating conditions (e.g., wide temperature fluctuations) of satellites and the components thereof, it is desirable to make and use components capable of enduring these operating conditions without detrimentally affecting their performance capabilities.
For example, one component used in communications satellites is a microwave resonator, which is a tuned, electromagnetic circuit that transmits energy at or near a resonant frequency. It can be used as a filter to remove electromagnetic signals of unwanted frequencies from input signals and to output signals having a preselected bandwidth centered about one or more resonant frequencies. Typical shapes used for such resonators include cylinders, rectangular bodies, and spheres. Side walls of a cavity defined by the resonator act as a boundary that confines the electromagnetic wave to the enclosed cavity. The frequency of the resonator is a function of the cavity dimensions. Accordingly, a change in temperature can cause the effective dimensions of the cavity to change by expansion or contraction of the resonator material which, in turn, causes the resonant frequency to change.
Microwave electronic components, such as microwave resonators, used in high-power communications satellites have been composed primarily of either conductive metal-plated aluminum or a conductive metal-plated iron-nickel Invar.TM. alloy (hereafter "Invar.TM."). Incorporation of conductive metal-plated aluminum or Invar.TM. into microwave electronic components, such as microwave resonators, has proven particularly troublesome because of their poor thermomechanical stability and/or their high weight.
Microwave electronic components, such as microwave resonators, are often made of aluminum due to its low specific gravity of about 2.7. However, aluminum exhibits poor thermal expansion resistance (i.e., a high coefficient of thermal expansion of about 20 ppm/.degree.C.). Thus, microwave electronic components made of aluminum exhibit poor thermomechanical stability and frequently must be tuned to maintain their resonant frequencies. A temperature compensation membrane, such as the one disclosed in U.S. Pat. No. 4,677,403, may be used to minimize the amount of tuning required to maintain the resonant frequency of an aluminum-based microwave electronic structure in spite of temperature-induced dimensional changes in the aluminum. The use of these membranes, however, results in decreased production efficiency and an increase in manufacturing costs. Furthermore, the added hardware required to tune the microwave electronic structure adds to the weight of the high-power communications satellite.
Invar.TM., an alternative to aluminum, exhibits a higher resistance to thermal expansion (i.e., a low coefficient of thermal expansion of about 1 ppm/.degree.C.). However, Invar.TM. is more dense (having a specific gravity of about 8.0) than aluminum, and therefore is undesirable for use in lightweight satellites.
It would be desirable to provide a lightweight microwave electronic structure, and a method for making the same, wherein the structure, and the material comprising the structure, exhibit thermomechanical stability, chemical durability, mechanical integrity, thermal shock resistance, and tailorable dielectric behavior suitable for use in lightweight spacecraft.